Several approaches are available: 1) test early convalescent sera for enzyme-linked immunosorbent assay (ELISA) yellow fever (YF) or DENV NS1 IgG or IgM antibodies.32 The presence of IgG DENV NS1 antibodies signifies the patient is experiencing a secondary DENV infection (primary vaccine failure), whereas the absence of these antibodies and the presence of YF IgG NS1 or DENV IgM NS1 antibodies suggests that the CYD vaccine provided the sensitizing infection. Here, CD40LG is linked to infection.